On the Structure of Roots. 



487 



tures, thrown off year after year, or dying with the stem in 

 annuals. Their woody structure differs very much from that of 

 the roots of Dicotyledons, so that they are easily known by ob- 

 serving a cross section ; but the cortical region and the growing 

 extremities differ little in the roots of the two classes. Tlie 

 principal characteristic of the roots of the Monocotyledons lies in 

 their woody central cord exhibiting no trace of distinct bundles 

 separated by medullary rays, but consisting of a central column of 

 wood, with its " ducts" or vascular structures lying on the outside, 

 at the region where the wood adjoins the cortical parenchyma. A 

 kind of cambium exists here also, although no annual rings are 

 ever formed, since it p- ^^_ 



is at this outer surface 

 of the woody region 

 that the root-buds ori- 

 ginate. 



The structure of the 



ordinary roots of her- 

 baceous Monocotyle- 



donous plants may be 



well examined in the 



onion. If we place an 



onion bulb over water 



in a long glass, like a 



hyacinth glass, it soon 



sends out a number of 



slender blunt-ended 



roots, of white colour, 



the tips only having 



a yellowish tinge. By 



placing longitudinal 



sections of one of them 



under the microscope, 



we can trace the mode 



of development of 



their roots. The ex- 

 treme point (fig. 14) 



is clothed by irre- 

 gularly - formed cells, 



loosely coherent, and 



evidently being partly 



thrown off by expansion of the structure beneath ; these cells 



pass laterally into a stratum of elongated cells, which clothe 



the whole external surface of the rootlet. In the interior of the 

 conical end of the root we find a mass of nascent cells, with 

 tiieir walls scarcely distinguishable, in a state of rapid multi- 



Perpendicular slice of the end of the rootlet of the onion, 

 magnified 100 diameters. 



